A semiconductor wafer of, for example silicon or gallium arsenide is produced in the form of a large diameter. This wafer is cut and separated (diced) into IC chips and is subjected to the subsequent die bonding step (also referred to as "mounting step") in which the IC chips are mounted on a lead frame for package. In this process, the semiconductor wafer undergoes dicing, cleaning, drying, expansion and pickup steps in the state of being attached to an adhesive sheet, and transferred to the subsequent die bonding step.
Adhesive sheets which are intended for use in the processing steps of wafers, from the dicing step up to the pickup step, are desired to have an adhesive force sufficient to retain wafers and/or chips thereon in the course from the dicing step up to the expanding step, but in the pickup step, they are desired to only retain an adhesion force of such an extent that no adhesive remains on the picked-up wafer and/or chips.
In the die bonding step, the picked up IC chips are fixed on IC chip mount zones (simply, mount zones) of a lead frame by means of an IC chip bonding adhesive such as an epoxy adhesive, a polyimide adhesive or a silver paste which is fed in the form of a viscous liquid to form an adhesive coat on the mount zones, followed by being subjected to the wire bonding step and resin mold step, thereby obtaining a semiconductor device. However, when the size of each of the IC chips is very small, it is difficult to uniformly apply an appropriate amount of adhesive with the use of the above liquid adhesive, so that the adhesive outflows the IC chips. On the other hand, when the size of each of the IC chips is very large, the use of the above liquid adhesive has encountered the problems such that the amount of the adhesive is short to thereby disenable bonding which ensures a satisfactory bonding strength. In recent years, the degree of integration of each semiconductor chip tends to increase, so that the chip tends to have an enlarged surface area and the wiring tends to become fine and a multilayer. On the other hand, the package in which the chips are accommodated tends to become miniaturized and thinner so that the mount on a printed wiring board can be conducted in high density. The obtained thin package with an enlarged surface area has encountered the problems such that the thermal shock and hot moisture resistances are poor and the package is likely to crack in the surface mount step, as compared with that of the prior art.
A film adhesive of a polyimide type resin having excellent heat resistance has been proposed for use in the bonding of IC chips to the lead frame. Further, it has been proposed to employ a dicing sheet comprising a substrate film and, detachably laminated thereto, such an adhesive for bonding of IC chips, which dicing sheet can simultaneously be used in dicing and die bonding.
However, the use of the polyimide adhesive in the above dicing sheet has encountered the limitation in employable substrate film because the solvent component of the polyimide adhesive has high boiling point and high polarity. Further, the above substrate film is generally so hard that the expansion thereof is not easy. Thus, it is difficult to enlarge the IC chip spacing with the result that erroneous operation is occasionally caused in the pickup of IC chips.